Recently, a personal computer has been adapted for multi-media, and color display has been generally used in a laptop type personal computer. In the laptop type personal computer or a desktop monitor, an STN type liquid crystal display or a TFT type liquid crystal display is mainly employed. In recent years, as the size of the liquid crystal display is increased, the TFT type liquid crystal display, which has an excellent image quality, has been predominated. Therefore, improvement of a viewing angle property has been eagerly required.
As a displaying mode of the TFT type liquid crystal display, an IPS mode or a vertical alignment (VA) mode is proposed and applied in addition to a conventional TN mode. A TN mode TFT liquid crystal display has characteristics in that cost of manufacture is reduced and efficiency of light utilization is high due to its optical rotatory mode. An optical compensation sheet developed in recent years improves a viewing angle property and is widely used.
For example, an elliptic polarizing plate for viewing angle compensation has a multilayer structure, and a typical example thereof has a structure a first transparent support (ordinarily, a cellulose triacetate film)/a polarizer/a second transparent support (ordinarily, a cellulose ester film)/an optically anisotropic layer formed by fixing the orientation direction of a liquid crystal compound in a specific direction/a transparent support C (for example, a stretched cellulose ester film) having an optically biaxial property, wherein the structure is obtained by forming the optically anisotropic layer directly or indirectly on the second transparent support, and then forming the transparent support C having an optically biaxial property on the optically anisotropic layer.
The above structure is obtained by providing the optically anisotropic layer formed by fixing the orientation direction of a liquid crystal compound in a specific direction on the transparent support C.
As the first and second transparent supports, a cellulose triacetate film with a thickness from 40 to 80 μm manufactured according to a solution cast film manufacturing method is employed. A polyvinyl alcohol film doped with iodine and stretched is employed as a polarizer, and the polarizer is inserted between the first and second transparent supports to obtain a polarizing plate.
The optically anisotropic layer is formed, for example, by providing a layer containing a liquid crystal compound having a polymerizable group on a support given an orientation capability through rubbing treatment and fixing the orientation direction by hardening the layer by UV light irradiation. The support is a transparent support having an optically biaxial property, and preferably a stretched cellulose ester film having a thickness of 40 to 150 μm.
A cellulose ester film, which when continuously manufactured in view of productive efficiency, is stretched in the transverse direction, is preferably employed as a transparent support having an optically biaxial property.
As a method of stretching the film in the transverse direction, a tenter according to a tenter method is generally employed. In the tenter, clips are arranged at equal intervals and molecules are oriented only in the transverse direction, and there is neither orientation in the longitudinal direction of the molecules nor dimensional variation in the longitudinal direction. As a result, the film thickness is reduced in inverse proportional to the stretching magnitude, resulting in an increase of a retardation in the thickness direction. The dimension in the longitudinal direction is regulated and orientation of the molecules in the longitudinal direction of the film is also regulated. In stretching the film in the transverse direction, there is problem in that molecules are oriented in the transverse direction, and a contractive force is generated in the transverse direction, which increases a plane orientation of the molecules and results in an increase of a retardation in the thickness direction (Rt).
In a method of stretching in the transverse direction other than the tenter method above, there is also problem in that unintended stretching in the longitudinal direction excessively increases a retardation in the thickness direction as compared with a retardation in the plane of the film.
As a film used in an LCD display, it is important that the film has not only physical uniformity but optical uniformity. A film having a low uniformity of a retardation in the plane or a retardation in the thickness direction or a low uniformity of dispersion of an optically delayed phase axis (hereinafter referred to also as an orientation angle) causes a light leak at black display, resulting in a serious defect practically.